Try to enable bluetooth coexistence mode on the WiFi chip...

In x-term:
This will have to be redone after each reboot of the device, if you find it to be a successful fix.

Update:
This (at least under some circumstances) gets set to 2 by default periodically, when BT and WLAN are both used. 0 = off, 1 = on, 2 = monoaudio. If you manually override, it's very possible your value will get changed by even before a reboot. I'm going to peek at the wl12xx sources to try to determine what exactly monoaudio mode does compared to just a value of on.

Update 2:
The difference between 1 (on) and 2 (monoaudio) is the wake up interval - for monoaudio, it is 5800ms longer (on is 500ms). The wake up interval appears to be a worst case value, and the BT radio should be given permission to transmit prior to that time when possible.